Additional email template variables for the Billable Invoice email

[MaximilianoRicoTabo]

• Get membership_level_name and membership_level_id from $order object and add to data object before call sendEmail function in pmproemail class
• Add level name to billable_invoice template

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• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Resolves #2891 .

How to test the changes in this Pull Request:

1. Apply this patch
2. Go to Memberships->Oders
3. Look into the Orders WP_List_Table and click email link to send en billing invoice from that order.
4. Check results.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

[dparker1005]

Other email templates, such as all the checkout templates, do not include the level name that was purchased. Reverting this change to stay consistent with those other email template defaults.

Suggested change

	'body' => __( '<p>Thank you for your membership to !!membership_level_name!! at !!sitename!!. Below is your invoice for order #: !!order_code!!</p>
	'body' => __( '<p>Thank you for your membership to !!sitename!!. Below is your invoice for order #: !!order_code!!</p>